Measurement of atmospheric sulfuric acid concentration using a water-based condensation particle counter

Gaseous sulfuric acid (SA, H2SO4) plays a key role in the heterogeneous distribution of urban atmospheric aerosol particle concentrations and composition. SA is formed in urban environments from photochemical reactions of gaseous precursors, specifically sulfur dioxide (SO2), emitted from vehicle exhaust and other fossil fuel combustion. Once in the atmosphere, SA drives atmospheric new particle formation (NPF), the process whereby gaseous precursors react to form stable aerosol particles. These particles grow to detectable sizes and are an important source of ultrafine particles (i.e., particles <100 nm in diameter). The rate at which new particles are formed in the atmosphere heavily depends on the concentration of SA, environmental conditions, and pre-existing particles. All these parameters often vary over short spatial and temporal scales in cities due to proximity to emission sources. As traffic and solar radiation vary considerably in the urban environment, the concentrations of SA and its contribution to aerosol size distributions via NPF will also vary. A complete understanding of why urban aerosol concentration and composition differ between small scales requires continuous measurements of sulfuric acid. This SBIR phase I project directly addresses the need to develop a network for high spatial and temporal measurements of urban NPF events and particle size distributions by developing a lower-cost instrument capable of concurrently monitoring gaseous sulfuric acid and sub-10nm particle concentrations. The proposed monitoring device, the Sulfuric Acid Dimethylamine-Reactive dual channel Condensation Particle Counter (SAD-R2CPC), combines a small nucleation flow reactor with a dual channel 1-nm diethylene glycol (DEG) water-based condensation particle counter (D2CPC). The flow reactor converts gaseous sulfuric acid into 1 nm particles detectable by the D2CPC. In Phase I, we will develop a bench-top prototype that addresses the current limitations of SAD-R2CPC and evaluate its performance in the field. The D2CPC is needed to simultaneously measure particles formed by nucleation from SA in the flow reactor and ambient particle concentrations. Data obtained from the second channel will improve SAD-R2CPC accuracy by minimizing interferences at high urban aerosol concentrations. ADI has extensive experience in developing and commercializing water-based CPCs. Task 1 will build and test a benchtop D2CPC with 1 nm cut points using DEG inlets coupled to water CPCs. Task 2 will combine the D2CPC with the nucleation flow reactor. The instrument will then be evaluated with laboratory-generated and ambient gaseous sulfuric acid and particle concentrations in Pittsburgh, PA. Phase II of this project will combine Phase I results with ADIĺs extensive experience to develop a compact, robust, and field-deployable version of the dual-channel SAD-R2CPC. Commercialization of this instrument is required for urban network monitoring. Other applications include monitoring contamination in industrial manufacturing processes and indoor air. The estimated cost of the final instrument is $50K, one tenth the cost of the currently used instrument known as the CIMS.